We exploit rules of class formation to identify the causal effect of increasing the number of immigrants in a classroom on natives’ test scores, keeping class size and quality of the two types of students constant (pure ethnic composition [PEC] effect). We explain why this is a relevant policy parameter although it has been neglected so far. The PEC effect is sizable and negative (16% of a standard deviation) on language and math scores. For first-generation immigrants, it is more negative (30% of a standard deviation). Estimates that cannot control for endogenous adjustments implemented by principals are instead considerably smaller.